1. Field of the Invention
The present invention relates to steering shaft supporting structure of a motor-driven steering assist apparatus.
2. Description of the Related Art
In a motor-driven steering assist apparatus of a rough road traveling vehicle such as a buggy vehicle or the like, as described in Japanese Patent Application Laid-open No. 2004-231011 (patent document 1), there is a structure in which a steering shaft to which a steering wheel is fixed is supported to a shaft support member fixed to a vehicle body side via a bearing. The steering shaft is connected to an input shaft of a motor-driven steering assist apparatus supported to the vehicle body side. This structure assists steering force applied to the steering wheel by a driver on the basis of a torque generated by an electric motor.
The motor-driven steering assist apparatus in the patent document 1 has the following features. A single unit body covered by a housing has an input shaft to which a steering shaft of a steering wheel side is connected, an output shaft to which a wheel side steering member is connected, and a torque sensor which is provided between the input shaft and the output shaft. An electric motor is driven in correspondence to a detected torque of the torque sensor, a worm gear is coupled to a rotary shaft of the electric motor, and a worm wheel is coupled to the output shaft and is engaged with the worm gear built-in. Further, the housing can be attached to a vehicle body frame, such as via a vehicle body side bracket.
In the motor-driven steering assist apparatus in patent document 1, the steering shaft is radial supported to a shaft support member fixed to a vehicle body side stay via a bearing. The motor-driven steering assist apparatus is also supported to a vehicle body side bracket, and can regulate an attaching attitude only in a range of a play of a bolt insertion hole provided in the vehicle body side bracket. Accordingly, if the steering shaft and an input shaft of the motor-driven steering assist apparatus are shifted in axes, an axial displacement can be hardly accommodated, and the steering shaft and the input shaft can not be connected.
When the driver pulls up the steering wheel in an upward direction or pushes down in a downward direction in correspondence to a circumstance in the case that that the vehicle to which the motor-driven steering assist apparatus in the patent document 1 is mounted travels, there may be circumstances such that a pull-up load or a push-down load is applied in an axial direction of the steering shaft. At this time, if the steering shaft is displaced in an axial direction on the basis of the loads in the axial direction, there is a risk that the displacement affects a torque sensor so as to produce an improper operating signal in the torque sensor or break the torque sensor.
Further, where the driver pulls up the steering wheel in an upward direction or pushes down in a downward direction in correspondence to a circumstance such that that the vehicle to which the motor-driven steering assist apparatus in the patent document 1 is mounted travels, a pull-up load or a push-down load is applied in an axial direction of the steering shaft. At this time, if the steering shaft is displaced in an axial direction on the basis of the loads in the axial direction, there is a risk that the displacement affects a torque sensor so as to produce an improper operating signal in the torque sensor or break the torque sensor.